1. Field of the Invention
The present invention relates to an oxygen absorbable laminate and a production method thereof. More specifically, it relates to an oxygen absorbable laminate which can effectively exert oxygen absorbability even when used in dried foods and the like; and a production method thereof.
2. Description of the Related Art
Heretofore, in order to prevent a degradation in the quality of foods and the like, there are known packaging containers such as packaging bags comprising a laminate film having resin layers to which an oxygen absorber utilizing an oxidation-reduction reaction has been added.
However, when such packaging bags are simply used for packaging dried foods and the like, it is difficult to supply, from the outside of the bags, water necessary to cause an oxidation-reduction reaction to exert oxygen absorbability. As a result, the oxygen absorbability is not effectively exerted.
Japanese Patent Application Laid-Open No. 051786/1997 discloses a packaging material for dried foods which is obtained by bringing a metal iron powder-containing sheet material into contact with water and allowing the resulting material to cure in a sealed system, and a production method thereof comprising winding up the metal iron powder-containing sheet material into the form of a coil after the contact with water, welding-sealing the coil-like material by using a material having oxygen barrier properties and allowing the material to cure.
Further, Japanese Patent Application Laid-Open No. 151783/1999 discloses a laminate for packaging which is obtained by preparing a laminate which comprises an external layer comprising continuous layers of a thermoplastic resin, an oxygen-barrierable intermediate layer, an oxygen absorbable intermediate layer and an internal layer comprising continuous layers of a moisture-resistant thermoplastic resin, winding up the laminate on a roll with 0.2 to 25 g/m2 of water uniformly present on its surface and allowing the rolled laminate to age in an oxygen-blocking atmosphere.
Further, Japanese Patent Application Publication No. 21083/1995 discloses a film product comprising an oxygen absorbable resin composition to which a hydrophilic filler has been added in order to supply a required amount of water.
More specifically, the film product is obtained by melting an oxygen absorbable resin composition to which 50 to 400 parts by weight of fine iron powders of at least 100 meshes, at least 2 parts by weight of sodium chloride fine powders of at least 100 meshes and at least 5 parts by weight of hydrophilic filler based on 100 parts by weight of a thermoplastic resin have been added to form the composition into a film, immersing the film in a water bath or humidified bath at room or elevated temperatures, removing water from the film and drying the film.
However, since the conventionally known oxygen absorbable film product has a smooth surface, it is difficult for the product to contain water in the amount required to exert oxygen absorbability effectively in a series of water incorporation treatments comprising applying water to the film, winding it up into a roll form and allowing the wound roll to age.
Further, when the conventional water-absorbed oxygen absorbable film product is simply used under low-moisture atmosphere, it fails to fully exert the oxygen absorbability for preventing a degradation in the quality of foods and the like, and this is particularly noticeable when the film product is used for packaging dried foods.
It is an object of the present invention to provide an oxygen absorbable laminate which can effectively exert oxygen absorbability even when used in dried foods and the like for which oxygen absorbability is not effectively obtained, and a production method thereof.
The present inventors have made intense studies on the problems and found that there is a correlation between the surface roughness of a surface of the oxygen absorbable laminate and the water content of the laminate after the water-applied laminate is rolled up and allowed to age and that the water content can be increased qualitatively and significantly by roughening at least one surface of the laminate.
Further, they have also found that in the case of an oxygen absorbable laminate in which the resins constituting an internal layer and an oxygen absorbable layer are ethylene resins, by making the resin density of the internal layer higher than the resin density of the oxygen absorbable layer, the laminate can exhibit stable oxygen absorbability even under low-moisture atmosphere.
According to the present invention, there is provided an oxygen absorbable laminate comprising an internal layer, an oxygen absorbable layer and an external layer, in which at least one surface of the oxygen absorbable laminate is roughened and subjected to a water incorporation treatment. As a result, water is retained in the oxygen absorbable layer, and the above problem can be solved.
That is, by roughening one or both surfaces of the oxygen absorbable laminate, the water content when water is applied to the laminate and the resulting laminate is rolled up and allowed to age is increased effectively. Consequently, the laminate can effectively exert oxygen absorbability even when used for dried foods and the like.
However, the present invention can also apply to any materials other than dried foods.
Further, the roughened surface preferably has an arithmetic average roughness (Ra) (in accordance with JIS B0601) of 0.4 to 20 xcexcm.
By having such roughness, the water content when water is applied to the laminate and the resulting laminate is rolled up and allowed to age is increased securely and quantitatively. Therefore, the laminate can effectively exert excellent oxygen absorbability over a long time period even when used for dried foods and the like.
Further, in the oxygen absorbable laminate of the present invention, the water content is preferably 0.5 to 25 g/m2 after water is applied to the laminate and the resulting laminate is rolled up and allowed to age.
Due to such water content, the laminate can quantitatively exert oxygen absorbability even when used for dried foods and the like.
The water content is specifically defined by the following expression (1).
W1=(W2xe2x88x92W3)/Axe2x80x83xe2x80x83(1) 
W1: water content (g/m2)
W2: weight (g) of oxygen absorbable laminate after aging
W3: weight (g) of oxygen absorbable laminate after drying (temperature: 70xc2x0 C., pressure: 1xc3x9710xe2x88x923 Pa or lower, time: 24 hours)
A: area (m2) of oxygen absorbable laminate
Further, in order to form the roughened surface, inorganic particles and organic particles, or either of these particles are preferably added to at least one of the internal layer, oxygen absorbable layer and external layer.
By adding such particles, a surface with the controlled roughness is formed easily and securely on a surface of the internal layer and/or a surface of the external layer, thereby increasing the water content of the oxygen absorbable laminate securely.
Further, a flattening layer is preferably provided between the oxygen absorbable layer and the external layer.
By providing the flattening layer, even when pits and projections are formed on a surface of the oxygen absorbable layer by the oxygen absorber particles contained therein or when the oxygen absorber particles are present on the surface of the oxygen absorbable layer, the flattening layer absorbs these particles, thereby not only preventing deterioration of the outer appearance but also facilitating the bonding between the oxygen absorbable layer and the external layer.
In the oxygen absorbable laminate of the present invention, the internal layer and the oxygen absorbable layer preferably comprise ethylene resins and the relationship between the resin density (d1) of the internal layer and the resin density (d2) of the oxygen absorbable layer is preferably d1 greater than d2.
By making the internal layer and oxygen absorbable layer of ethylene resins as constituents of the oxygen absorbable laminate and having the relationship d1 greater than d2 between the resin density (d1) of the internal layer and the resin density (d2) of the oxygen absorbable layer, the moisture retainability can be improved by preventing water entered the oxygen absorbable layer from dissipating to the outside of the layer and the permeability of oxygen to the above oxygen absorbable layer via the internal layer can be improved. This oxygen absorbable laminate can then exert more excellent oxygen absorbability.
In addition, in the oxygen absorbable laminate of the present invention, the density (d1) of the ethylene resin constituting the internal layer is preferably 0.915 g/cm3 to 0.950 g/cm3. This resin density allows more remarkable improvement in the above properties.
Further, in the oxygen absorbable laminate of the present invention, a water-vapor permeability coefficient (H1) of the ethylene resin constituting the oxygen absorbable layer is preferably higher than or equal to a water-vapor permeability coefficient (H2) of the ethylene resin constituting the internal layer.
By satisfying such a relationship between the water-vapor permeability coefficients, water entered the oxygen absorbable layer is blocked by the internal layer, thereby preventing the dissipation of water from the oxygen absorbable layer to the outside thereof more effectively.
Further, in the oxygen absorbable laminate of the present invention, an oxygen permeability coefficient (O1) of the ethylene resin constituting the oxygen absorbable layer is preferably higher than or equal to an oxygen permeability coefficient (O2) of the ethylene resin constituting the internal layer.
By satisfying such a relationship between the oxygen permeability coefficients, oxygen reaches the inside of the oxygen absorbable layer effectively, thereby exerting excellent oxygen absorbability.
Further, according to the present invention, there is provided a production method of an oxygen absorbable laminate comprising an internal layer, an oxygen absorbable layer and an external layer, comprising the steps of:
roughening at least one surface of the oxygen absorbable laminate,
applying water to the above oxygen absorbable laminate,
winding up the above water-applied oxygen absorbable laminate into a roll form, and
allowing the above water-applied rolled oxygen absorbable laminate to age.
The method allows easy production of the oxygen absorbable laminate which has good moisture retainability in the oxygen absorbable layer and effectively exerts oxygen absorbability even when used for packages of dried foods and the like.
Further, when the production method of the oxygen absorbable laminate of the present invention is carried out, water is preferably applied to the oxygen absorbable laminate after the laminate is cured.
By producing the oxygen absorbable laminate in such a manner, the peeling of the oxygen absorbable laminate is securely prevented when water is applied to the surface of the laminate.
Further, when the production method of the oxygen absorbable laminate of the present invention is carried out, the roughened surface preferably has an arithmetic average roughness (Ra) (in accordance with JIS B0601) of 0.4 to 20 xcexcm.
By producing the oxygen absorbable laminate in such a manner, the water content is increased securely and quantitatively after water is applied to the laminate and the laminate is allowed to age. Therefore, the laminate can effectively exert excellent oxygen absorbability over a long time period even when used for dried foods and the like.
Further, when the production method of the oxygen absorbable laminate of the present invention is carried out, inorganic particles and organic particles or either of these particles are preferably added to at least one of the internal layer, oxygen absorbable layer and external layer to roughen a surface of the above oxygen absorbable laminate.
By adding such particles, the surface of the oxygen absorbable laminate can be roughened easily without subjecting the surface to a mechanical treatment and the like.